Ten Heads

This is most easily done in the level "Direwolves" within "Dark Forest" during the campaign. Just make sure that you have a single arrow equipped to your ballista, and select the "Casual" difficulty.

Once the level starts, just position your ballista, and wait for the wolves and time the shots so that you get only headshots. If you kill all 10 enemies in a row with headshots, you will unlock the achievement!